[msluyter]

I was going to write up pretty much the same analysis of the Em->X progression. I think you aptly covered it. Interestingly, though, as you say Em->C sounds relatively static, I bet you see a lot more C->Em than the reverse (which should suffer from the same problem), and I'm hard pressed to say why exactly.

[colanderman]

My guess would be that it's a precursor to an Em->F or Em->Am transition; i.e., if you wanted to transition to a minor mode convincingly from C, you'd toss the Em (or even E7) in there before hitting Am. Example: Santeria by Sublime (I-III-vi-V)

[Goladus]

I can't speak for pop music, but you can move from Em to C/E to F, which is basically a delayed resolution of the E to F, but the C Major is definitely there.